More particularly, the invention relates to the field of visual inspection of tires in which the image of a tire to be inspected is compared with a reference image of this tire. The comparison of these two images makes it possible, by analysis of the differences, to determine, at a given point on the image, the variations in the value of a physical magnitude assigned to this point and originating from a generally digital image sensor. The physical magnitudes measured by the sensor may relate to the color, to the brightness, to the texture of the object observed or else to the height of the points relative to a reference surface, when equipment capable of creating a three-dimensional image is used.
These variations are analyzed with the aid of algorithms adapted to determine the degree of conformity of the tire to be evaluated, and to decide whether this tire can be considered to be compliant or whether it must be directed toward a treatment center.
One of the difficulties to be overcome for carrying out this type of inspection consists in having an image that is as close as possible to the “ideal” model of the type of tire to be inspected while taking account of the variability of acquisition.
This ideal model may originate for example from the digital image used in the design of the tire or in the production of the mold, but it then becomes difficult to integrate the shadow zones associated with the nature and with the position of the lighting elements, to take account of the presence and the shape of elements such as the teats formed by the vents, or to retrieve, by calculation, the variations in the values originating from the physical magnitudes measured, and associated with the nature of the generally black rubber materials, as a function of the chosen lighting angles, or else of the calibration of the sensors.
Another possibility consists in taking as a reference the image of a tire that is real and considered to be “ideal”. In this case also, it is necessary to overcome other difficulties, such as the variation in brightness or in texture of the surface to be evaluated, the evolution of which is associated with the surface state of the mold, but also, the reproducibility of the lighting elements between the images or else the differences in positioning of the tire between the production of the two images.